• THE , HISTORY OF THE EARTH.*
A DISPUTE has long raged between physicists and geologists as to the possible age of the earth. The geologists in tracing the wonderful history which is written in the strata. of the earth's crust—a literal sermon in stones—have been inclined to exaggerate the vast length of time during which that record must have been in progress. The formation of the sedimen- tary rocks, laid down under water on the, beds of ancient seas or rivers, must have occupied an almost unthinkably vast period. We have only to look at the slow process of sedimen- tation as it goes on to-day, and compare this with its results in the past. We can measure the amount of debris Which a river brings down from the land to spread along the sea-floor every year, and we can argue from this bow many thousands or millions of years must have been taken to buildup the strata, perhaps fifty miles in total thickness, which com- pose the upper portions of .the . earth's crust. If • we look at the imposing cliffs of solid chalk which line our Southern - coast and form so conspienous and bea-utiful a sign of home .to the returning traveller, and remember that the chalk, of which they are. composed consists entirely of the shells and skeletons of tiny marine organisms which can only be seen through a microscope, we, begin to realise how long it must haye taken for. these minute- relics, slowly but steadily raining down upon the floor of the ancient sea, to have built up those solid deposits of chalk, known to be as much as fourteen thousand feet in thickness. The biologist, again, contrilputes his argument towards proving the gigantic. age of the earth, as measured in terms of hinnan chronology, though in the whole cosmic drama it..may be small enough. Since we have,accepted Darwin's . explanation ef the origin of species, we have to allow an .immense time.for the gradual development of existing forms of animal life from the earliest protozoa. As Professor Sollas points out in his very interesting lecture on, the age of the ,earth, the oldest fessil- bearing rocks would be overlaid, if the various denuding agencies had not been at work, by a thickness. of thirty-four miles of sediment. The fossils which they contain represent nearly all classes of life now known, with the exception' of, the vertebrates. Thus the evolution of the vertebrata alone is known to have occupied a period of time represented' by the formation of thirty-four miles of sediment. How thrich • * The' Age of the Earth. By W. J. Sanas. London: T. Fisher Unwio. [10s. 6d. net.]—(2) Ice or. Water ? By Sir Henry Howorth. Vols. I. and II. London : Longmans and Co. [32s. net.]—(3) Extinct Anintals. By E..-Bay Lankester. London : A. Constable and Co. [7s. 6c1. net.]—(4) An Introduc- tion to ,Geology. By J. E. Idarr. Cambridge at the University Press.' [3s. riet:3—(5) The Founder*' of Geology. •By Sir Archibald Gaikie. London: Macmillan and Co. [10s. net.]
greater must have been the interval required for the evolution of the whole organic world ! The human mind, dwelling on such considerations, seems at times to have been affected by a surexcitation of the imagination, and con- sequent paralysis of the understanding, which led to a refusal to measure geological time by years at all, or to reckon by anything less than aeons.
The poetical tendency shown at times by both geologist and biologist to think in continents and aeons, combined with the consequent refusal to undergo the base prosaic drudgery of actual calculation, has met with severe condemnation from devotees of the more exact sciences. Lord Kelvin, for instance, found it necessary some thirty years ago to check geologists in their dealings with infinite and immeasurable ages. The late Sir Andrew Ramsay saw nothing very absurd in assuming that the history of the earth might have occupied ten thousand million years. But since the aid of more exact science has been brought to the assistance of geology, wide estimates of this kind are outside the mark. It is true that even with the aid of physics and astronomy we cannot fix the dates of the various epochs in the earth's history with the same accuracy as those of the Norman Conquest or the French Revolution. But we can get very much nearer the mark than the mere geologist can take us. Professor Sollas deals with the various estimates of the age of the earth derived from various lines of investigation. The earth may be regarded in the light of a clock, by examining which the skilful watchmaker can generally tell with fair accuracy when it was last wound up. We know that the speed of the earth's rotation is steadily diminishing, and we can calculate backwards to find out when it must have consolidated in its present shape under the physical laws which affect the form of rotating bodies of liquid. The earth is losing heat, and by calculating the rate at which this goes on we can make a fairly close guess at the time when it must have been so hot as to be liquid throughout. The sun is steadily contracting, and the comparatively modern theory which shows that this contraction keeps up the solar emission of heat enables us to calculate just how long the sun has been capable of vivifying the earth. Sir George Darwin has shown us how to measure the time which has elapsed since the moon was torn away from the earth by the strain of too quick rotation. By such methods we can estimate the time which has elapsed since the earth solidified, and none of these estimates has any- thing in common with the vast indefinite ages of the early geologists. Thus Sir George Darwin estimates that about fifty-six million years have elapsed since the moon came into existence. Lord Kelvin estimates that twenty to forty million years have elapsed. since the surface of the earth began to consolidate. The recent discovery of the univer- sality of radio-activity has somewhat upset this calculation by introducing a possible new factor. But there is no doubt whatever that the whole age of the earth, since its surface began to be subjected to the moulding influences of what we call geological agencies, cannot be more than fifty or sixty million years. Professor Sollas shows that all the best modern estimates of the earth's age can be brought into fair agree- ment. Probably the stratified rocks, which represent the work Of natural agencies still in operation, have taken something like twenty-six million years to form, and it is within this period that we must place the development of life on the earth. Geological chronology still lacks the minute accuracy for which some students have wished, but it shows great advance upon the wild language of the early geologists, who held with Ramsay that they might roam in fancy through thousands of millions of years. The more we know of the making of the earth the more wonderful it seems, and the reverent student of the great drama of evolution is led to still greater admiration of the wonderful adjustment of the universe as he comes to understand it more and more closely.
In two fat volumes, Ice or Water ? which are only the first instalments of a portentous work, Sir Henry Howorth.under- takes to prove that the Glacial Period never existed. He has devoted a great deal of study and ingenuity to this attempt, and, so far as we can follow his argument, he desires to replace the Glacial Period by a great deluge. Unfortunately for his labour, it requires something more than ingenious dialectics to establish a scientific theory, and Sir Henry Howorth has little more to offer.
Professor Laukester's Extinct Animals is a reproduction of the lectures which he gave to children at the Royal Institute during the Christmas holidays of 1903-4. The book is admirably adapted to give the young reader a taste for such inquiries, and a good idea of the kind of creatures which lived in the world before human history began. It is illustrated by numerous excellent photographs, which are the next best thing to the study of the actual fossils themselves.
Dr. Marr, the President of the Geological Society, in his Introduction to Geology succeeds in his aim of explaining the scope and methods of this science without an excess of detail. The nature of the rocks which make up the earth's crust, the methods by which they have been formed and modified, and the geological record written in them are simply and lucidly explained in this excellent primer.
Sir Archibald Geikie has enlarged and rewritten the lectures which he published in 1897, and has thus produced a complete history of geological science clown to the middle of the nineteenth century. His well-known Lo wer of lucid exposi- tion is admirably illustrated in this vorime, The Founders of Geology, which fills a gap in our scientific literature, and traces the gradual progress of earth-knowledge from Lucretius to Lyell.
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